It is proposed to study phase equilibria and mechanical properties in Mg-Zn-RE (rare earth: Nd, Ce, Sm) alloys. Zn additions were found to provide an avenue for heat treatment analogous to GP zone formation in Al-Cu alloys. Results have shown that RE additions dramatically improve mechanical properties, but most research so far has been by trial and error. The proposed study adopts a computational materials design approach to optimize Mg-Zn-Nd(Ce, Sm) alloys for the best combination of mechanical properties in the as-fabricated (cast and extruded) and heat-treated conditions. The proposed study (1) produces thermodynamic descriptions of the Mg-Zn-Nd(Ce, Sm) systems; (2) carries out directional solidification of representative alloys to characterize structures and compositions of the phases formed and solidified microstructures, in the context of the phase diagrams obtained; (3) measures the mechanical properties of selected alloys in the solidified state and after specific thermal treatment. Bulk mechanical properties (yield strength, ultimate tensile strength, hardness, ductility) are characterized. Nanoindentation will be used to relate bulk properties to processes going on at the micro-scale and to examine the mechanical properties of the individual phases. The intellectual merit of this research includes the use of computational thermodynamics coupled with key experiments to rapidly obtain thermodynamic descriptions of Mg-Zn-RE. This approach differs from traditional ones that rely only on experiments, which are tedious and ineffective for multi-component systems. The thermodynamic descriptions obtained can be used to provide appropriate thermodynamic quantities as input for kinetic models, for predicting microstructures and, ultimately, mechanical properties.
NON-TECHNICAL SUMMARY: Magnesium is the lightest structural metal, and researchers worldwide are motivated by the promise of improved fuel economy and reduced environmental impact to develop magnesium alloys for replacing steel and aluminum in automotive applications. To address this goal the present research uses a computational thermodynamics approach to develop a new class of promising magnesium-zinc-rare earth (Nd, Ce, Sm) alloys in a manner that is more efficient and accurate than has been achieved previously. The mechanical properties (strength) of the alloys will be tested against the micro- and nanoscale properties of the phases inside the alloys, so that the knowledge gained will lead to better understanding for future development of other kinds of alloys. This work involves collaboration among researchers from the University of Wisconsin and General Motors. The broader impact of the proposed work includes an opportunity for young people - future scientists and engineers - to experience working on a broad, fundamental problem with both academic and industrial importance and under the supervision of both industrial and academic advisors. The students will gain the invaluable experience of working on a portion of their research in an industrial environment, i.e. at the General Motors R&D Center.
